Lemon sole

Microstomus kitt


Technical report
Published by

Marine and Freshwater Research Institute, Iceland

Published

6 June 2025

Key signals

  • Survey biomass was high during 2004–2010, peaking in 2006, then declined, but has remained variable and is currently at a similar level to 2002.

  • Recruitment has been strong since 2021, and the index for larger lemon sole is high.

  • Length distributions in both surveys and commercial catches have remained relatively stable over time.

  • Fishing mortality (F) has remained below the FMSY proxy since 2015.

  • Landings from bottom trawls are increasing.

  • The lemon sole stock in Division 5.a is considered stable.

General information

Lemon sole is found all around Iceland, but the highest abundance and the main fishing grounds are off the west and southwest coast. Lemon sole is a demersal and shallow water species, mainly found on a sandy or gravel substrate and occurring mostly at 20-200 m depths. Females grow larger than males; only a small proportion of males become larger than 35 cm, whereas females rarely grow larger than 40 cm. Size at sexual maturity differs between the sexes. On the main spawning grounds off the south coast, large males are rarely found and about half of the males reach maturity at the length of 13 cm, while females reach that level at 24 cm length.

Fishery

Lemon sole landings in Icelandic waters show three main peaks: before World War II and during the post-war period (dominated by foreign fleets), and from the mid-1990s onward (almost exclusively by Icelandic vessels) (Figure 1). Landings declined sharply after the 1970s, likely due to the expansion of Iceland’s EEZ and reduced foreign access. Catches increased steadily through the 1980s and 1990s, peaking between 2005 and 2009 at over 2,500 tonnes. Following this peak, landings declined sharply, falling to around 1,000 tonnes in the most recent years.

The main fishing grounds for lemon sole are located along the west and south coasts of Iceland (Figure 2; Figure 3). Very little catch is recorded from the north and east coasts. In recent years, a gradual shift in fishing activity toward the northwest has been observed. Since 2000, the spatial distribution of the Icelandic lemon sole fishery has remained relatively stable, with approximately 85% of the catch taken from the south, southwest, and west coasts (Figure 3). Most catches during this period have been taken at depths of 40–120 meters (Figure 4). In recent years, the share of the catch taken at 40–80 m depth has increased, while catches at 80–120 m have declined—although this pattern reversed again in the last two years.

Lemon sole is caught primarily in demersal seine gear and as bycatch in demersal trawls, which together account for over 95% of landings in most years (Figure 5; Table 1). This proportion has remained relatively stable, although the share caught by trawlers has increased, exceeding 60% in each of the last three years. Demersal seiners typically operate at 40–80 m, whereas lemon sole caught in trawls are mostly taken at 80–120 m. The increase in trawl landings coincides with a deeper depth distribution of the catch.

Between 54 to 116 trawlers and 33 to 79 demersal seiners have reported lemon sole catches since 2000 (Table 1). Over the past decade, the number of demersal seiners has declined from 41 in 2014 to 33, while the number of trawlers has fluctuated around 60. The number of vessels accounting for about 95% of the lemon sole catch declined from approximately 135 in 1996 to just under 50 in 2015, with a slight increase in subsequent years (Figure 6). The reduction in vessel numbers between 1996 and 2007 occurred despite relatively high catches, whereas the decline from 2009 to 2014 coincided with a drop in total landings.

Figure 1: Lemon sole in 5a. Recorded landings since 1903.
Figure 2: Lemon sole in 5a. Spatial distribution of catches by all gears according to Icelandic logbooks.
Figure 3: Lemon sole in 5a. Spatial distribution of lemon sole catches by area as recorded in Icelandic logbooks.
Figure 4: Lemon sole in 5a. Depth distribution of lemon sole from demersal seine and trawl according to Icelandic logbooks.
Figure 5: Lemon sole in 5a. Landings in tonnes and percent of total by gear and year.
Table 1: Lemon sole in 5a. Number of Icelandic vessels landing lemon sole, and all landed catch divided by gear type (in tonnes).
Year Nr. Demersal Seine Nr. Bottom Trawl Nr. Other Demersal Seine Bottom Trawl Other Total Catch
2000 74 116 135 817 593 32 1 442
2001 69 103 149 865 484 25 1 374
2002 64 95 112 553 372 25 950
2003 79 93 129 772 447 26 1 245
2004 79 94 105 1 567 611 30 2 208
2005 71 97 95 1 612 873 24 2 509
2006 72 85 98 1 452 1 219 22 2 693
2007 70 88 97 1 191 1 449 23 2 663
2008 63 78 108 1 283 1 319 34 2 636
2009 61 77 106 1 445 1 145 43 2 633
2010 50 72 98 993 894 81 1 968
2011 47 65 109 1 164 610 126 1 900
2012 48 68 110 1 099 424 91 1 614
2013 45 69 86 1 303 380 84 1 767
2014 41 60 85 859 292 51 1 202
2015 41 64 92 1 237 475 60 1 772
2016 37 65 81 1 108 563 64 1 735
2017 39 70 83 953 465 33 1 451
2018 38 62 71 1 085 592 37 1 714
2019 36 62 69 892 533 23 1 448
2020 35 58 46 649 459 21 1 129
2021 33 62 59 725 707 15 1 447
2022 37 61 53 557 697 1 1 255
2023 33 55 54 337 677 1 1 015
2024 32 54 50 439 698 1 1 138
Figure 6: Lemon sole in 5a. Number of vessels (all gear types) accounting for 95% of the total catch annually since 1994. Left: Plotted against year from 1994. Right: Plotted against total catch. Data from the Directorate of Fisheries.

Catch per unit effort and fishing effort

Catch per unit effort (CPUE) estimates for lemon sole in Icelandic waters are not considered reliable indicators of stock abundance. This is due to the lack of standardization for changes in fleet composition, technical advancements, and differences in gear setup, among other factors.

Non-standardized CPUE for demersal seine (kg/set) is calculated from sets where lemon sole made up more than 10% of the catch and was present in the haul. Between 2000 and 2012, CPUE under these conditions averaged around 200 kg/set. From 2013 to 2024, CPUE increased and fluctuated between 250 and 370 kg/set (Figure 7).

In demersal trawl, CPUE (kg/hour) for hauls where lemon sole comprised more than 10% of the catch gradually increased from approximately 50 kg/hour in 2000–2002 to a peak of around 120 kg/hour in 2015–2017. In the past three years, CPUE has declined, reaching about 80 kg/hour in 2024 (Figure 7).

Figure 7: Lemon sole in 5a. Catch per unit of effort in the most important gear types. The dashed lines are based on locations where more than 10% of the catch is lemon sole and solid lines on all records where lemon sole is caught.

Sampling overview

An overview of the number of samples is provided in Figure 8, and gear-specific sampling coverage in 2024 is shown in Figure 9.

Biological sampling from the main gears (demersal seine and bottom trawl) in commercial lemon sole catches is generally considered acceptable. The sampling adequately covers the spatial and temporal distribution of catches from the main gears.

Table 3 and Table 2 present length and otolith sampling data from commercial lemon sole catches from 2000 to 2024. In general, over 90% of sampled otoliths have been age-read.

Figure 8: Lemon sole in 5a. Ratio of samples by month (bars) compared with landings by month (solid black line) split by year and main gear types. Numbers above the bars indicate number of the samples by year, month and gear.
Figure 9: Lemon sole in 5a. Fishing grounds in 2024 as reported in Icelandic logbooks (colours) and positions of samples taken from landings (asterisks) by main gear types.

Length distribution of landed catch

The relative length distribution of landed lemon sole has remained fairly stable, with a slight shift toward larger fish observed since 2017 (Figure 10).

Table 2: Lemon sole in 5a. Number of samples and length measurements from landed catch.
Year
Bottom Trawl
Demersal Seine
Num. samples Num. lengths Num. samples Num. lengths
2000 10 940 21 1 642
2001 4 350 12 1 019
2002 6 525 16 1 372
2003 5 460 21 1 696
2004 6 555 56 6 007
2005 12 1 107 26 2 617
2006 28 2 246 39 3 545
2007 35 3 045 56 5 423
2008 33 2 684 37 3 043
2009 18 1 552 42 3 266
2010 25 2 002 26 2 033
2011 15 1 070 45 4 048
2012 15 1 207 42 3 304
2013 9 613 44 3 295
2014 8 563 26 2 612
2015 19 1 649 32 2 758
2016 18 1 526 28 2 070
2017 19 1 813 25 1 875
2018 13 1 296 26 2 259
2019 24 1 662 24 2 040
2020 17 1 326 14 1 045
2021 37 3 895 20 1 270
2022 27 2 423 13 998
2023 36 2 855 4 234
2024 31 2 779 15 894
Figure 10: Lemon sole in 5a. Commercial length distributions by gear and year

Age composition of landed catch

The age composition of landed lemon sole is primarily made up of 6–9 year old fish (Figure 11). In recent years, however, the proportion of younger age groups has declined, while the presence of fish aged 10 years and older has become more noticeable.

Table 3: Langlúra. Number of samples and otoliths collected from landed catch.
Year
Bottom Trawl
Demersal Seine
Num. samples Num. otiliths Num. samples Num. otiliths
2000 10 275 21 525
2001 4 100 12 275
2002 6 175 16 376
2003 5 100 21 425
2004 6 75 56 1 100
2005 12 300 26 576
2006 28 375 39 725
2007 35 500 56 925
2008 33 500 37 725
2009 18 275 42 900
2010 25 506 26 575
2011 15 300 45 900
2012 15 361 42 925
2013 9 200 44 899
2014 8 200 26 500
2015 19 420 32 700
2016 18 425 28 675
2017 19 475 25 550
2018 13 275 26 545
2019 24 400 24 470
2020 17 350 14 345
2021 37 840 20 350
2022 27 485 13 245
2023 36 600 4 80
2024 31 540 15 238
Figure 11: Lemon sole in 5a. Catch at age from the commercial fishery in Iceland waters. Bar size is indicative of the catch in numbers and bars are colored by cohort.

Survey data

The Icelandic spring groundfish survey (hereafter spring survey, IS-SMB), conducted annually in March since 1985, covers the main distribution area of the lemon sole fishery. An additional autumn groundfish survey (hereafter autumn survey, IS-SMH) has been conducted since 1996, except in 2011. The spring survey is considered more reliable for detecting changes in biomass and abundance than the autumn survey. However, it may not adequately cover the main recruitment grounds, as lemon sole nursery areas are believed to occur in shallow habitats not sampled by these surveys.

Figure 12 shows trends in biomass indices and a recruitment index based on the abundance of lemon sole ≤20 cm. The total biomass index, as well as the biomass of lemon sole >30 cm (representing the harvestable portion of the stock), has declined with fluctuations since peaking in 2006. As of this year, the biomass index is at a level similar to that observed in 2002, prior to the peak (Figure 12).

The index for lemon sole >39 cm rose significantly between 2000 and 2004, again in 2016, and has since fluctuated around that level, reaching its highest value in the most recent spring survey.

The juvenile abundance index (≤20 cm) from the spring survey declined after peaking in 2011 but increased again in 2021, reaching its highest recorded value in spring 2024.

The autumn survey shows broadly similar trends to the spring survey. However, the juvenile abundance index from the autumn survey exhibits a slightly different pattern — though it also increased over the past three years, it remains at lower levels than observed in the spring.

In the spring survey of 2025, lemon sole was primarily caught on the main fishing grounds in the west and northwest of Iceland, with a notable amount also recorded in deeper waters off the southeast coast. In contrast, the coldest waters off the northeast and east coasts are almost devoid of lemon sole (Figure 13).

The spatial distribution of lemon sole in the spring survey has remained relatively stable since the survey began in 1985. However, catches in the northwest have increased over the past three years (Figure 14). The rise in estimated biomass after 2002 occurred more or less evenly across all survey areas. The spatial distribution observed in the autumn survey of 2024 closely resembles that of the most recent spring survey, except that the northwest component is less prominent in the autumn (Figure 13). In recent autumn surveys, most lemon sole have been caught in the west, southwest, and northwest regions of the country (Figure 14).

Similar to the commercial catch, the size distribution of lemon sole in the spring survey has remained stable over time, with little variation in length composition (Figure 15). An increase in recruitment is visible in the spring survey beginning in 2020, with a small peak also observed in last year’s autumn survey.

Figure 12: Lemon sole in 5a. Indices in the spring survey (SMB) 1985 and onwards (line shaded area) and the autumn survey (SMH) (point ranges).
Figure 13: Lemon sole in 5a. Location of lemon sole in the most recent spring (SMB) and autumn (SMH) surveys, bubble sizes are relative to catch sizes, grey x indicate all stations sampled during the corresponding survey.